1. Field of the Invention
This invention relates to a linear drive system including linear motor essentially consisting of a permanent magnet and an armature coil.
2. Description of the Related Art
As one of the conventional drive system of the type described above, the invention disclosed in JP-A-2001-352744 is known. The conventional drive system is illustrated in FIGS. 13, 14. As is clear from these diagrams, a conventional linear drive system has a pair of parallel track rails 2 laid on a bed 1 made of a ferromagnetic material. A plurality of sliders 3 are movably provided on the track rails 2. A movable table 4 made of a ferromagnetic material is fixed to the sliders 3 so that the movable table 4 can reciprocate on the track rails 2.
A plurality of permanent magnets 5 are arranged on the face of the movable table 4 facing the bed 1 in a direction of movement of the movable table 4. Each of the permanent magnets 5 is formed in a rectangular plate shape. Each of the permanent magnets 5 has a north pole and a south pole at its respective sides in the up-down direction in which the movable table 4 is located on the permanent magnet 5. The adjacent permanent magnets 5 arranged in the direction of movement of the movable table 4 are opposite in polarity to each other. The movable table 4 is formed of a ferromagnetic material in order that the movable table 4 can function as a magnet yoke through which a magnetic flux flows from the permanent magnets 5.
On the bed 1, on the other hand, a recess 6 is formed in a portion between the pair of track rails 2 and extends in the longitudinal direction of the track rails 2. The recess 6 is covered with a coil substrate 7. In turn, a plurality of armature coils 8, which take the form of three-phase cureless coils, is installed under the coil substrate 7. The armature coils 8 are arranged in a row along the longitudinal direction of the recess 6. Upon the application of three-phase current to the armature coils 8, the electromagnetic interaction between the magnetic fluxes of the permanent magnets 5 and the three-phase current passing through the armature coils 8 results in the movement of the movable table 4 with the sliders 3 on the track rails 2. Accordingly, the permanent magnets 5 and the armature coils 8 form a linear motor. The bed 1 is formed of a ferromagnetic material in order that the bed 1 can function as a yoke of the armature coils 8.
In a conventional system as described above, upon the application of current to the armature coils 8, the movable table 4, together with the sliders 3, is moved on the track rails 2 by the electromagnetic interaction between the magnetic fluxes of the permanent magnets 5 and the three-phase current passing through the armature coils 8. Recently, the drive system has been increasingly required to increase in speed and response. In order to address these needs, an increase in the thrust of the drive system is necessary. To increase the thrust, a reduction in the weight of the movable table 4, an increase in the magnetic force of the permanent magnet or an increase in the electric current applied is required.
The reduction of the weight of the movable table 4 needs a reduction in the thickness thereof. However, a reduction in the thickness of the movable table 4 involves inferiority in rigidity, leading to a warp in the movable table 4. Such a warp in the movable table 4 gives rise to degradation in performance.
Regarding another possible means of increasing the magnetic force of the permanent magnet 5, the magnets currently used for systems as described above have a significantly strong magnetic force. This makes it close to impossible under the present circumstances to use any magnet with a stronger magnetic force. An example of possible approaches for obtaining a strong magnetic force is an increase in size of the permanent magnet 5 or the armature coil 8. However, an increase in size of such a component involves an increase in mass of the component. Accordingly, even if the thrust can be increased, the increased thrust is wasted for the increased mass. In consequence, the increase in the magnetic force of the permanent magnet 5 is limited under the present circumstances.
As the electric current applied to the armature coils is increased in order to obtain a strong magnetic force, the amount of heat generated increases. If the amount of heat generated increases, the armature coils 8 may burn due to this heat, for example. In consequence, the increase in electric current applied is limited.
In either case, the conventional drive systems have limits for the achievement of speedups or fast response.